Liquid crystal display devices are used in watches, calculators, various measuring instruments, automobile panels, word processors, electronic notepads, printers, computers, televisions, clocks, advertizing boards, etc. Representative examples of the types of liquid crystal displays are a twisted nematic (TN) type, a super twisted nematic (STN) type, a vertical alignment (VA) type that uses thin film transistors (TFTs), and an in-plane switching (IPS) type. The liquid crystal compositions used in these liquid crystal display devices are required to be stable against outer factors such as moisture, air, heat, and light, stay in a liquid crystal phase in a temperature range as wide as possible about room temperature, exhibit a low viscosity, and operate at low driving voltage. A liquid crystal composition is constituted by several to several tens of compounds in order to optimize the dielectric anisotropy (Δ∈), refractive index anisotropy (Δn), etc., for individual liquid crystal display devices.
In VA-type displays that are widely used in liquid crystal televisions and the like, a liquid crystal composition having a negative Δ∈ is used. Meanwhile, for all drive types, low-voltage driving, high-speed response, and a wide operation temperature range are desired. In other words, the absolute value of Δ∈ is required to be high, the viscosity (η) is required to be low, and the nematic phase-isotropic liquid phase transition temperature (Tni) is required to be high. Also, there is need to adjust Δn of the liquid crystal composition to be in an appropriate range that suits the cell gap since the product Δn×d of Δn and a cell gap (d) is set. Moreover, high-speed response is important for liquid crystal display devices applied to televisions and the like and thus a liquid crystal composition having a low rotational viscosity (γ1) is desired.
Multi-domain vertical alignment (MVA)-type liquid crystal display devices which are a type of VA displays with improved viewing angle characteristics are now widely used. In this liquid crystal display device, projecting structures are formed on a substrate to divide a pixel so that the liquid crystal molecules are aligned in plural directions. A MVA liquid crystal display device is advantageous in terms of viewing angle characteristics but has a problem in that liquid crystal molecules respond at different speeds between the portion near the projecting structure on the substrate and the portion remote from the projecting structure and thus the overall response speed has been insufficient due to the liquid crystal molecules that are remote from the projecting structure and slow in response. There has also been degradation of transmittance caused by the projecting structure. In order to address these issues, polymer sustained alignment (PSA) liquid crystal display devices (an example of which is a polymer stabilized (PS) liquid crystal display device) have been developed to provide uniform pretilt angles within each domain of a pixel without forming non-transmitting projecting structures in a cell, which is different from a typical MVA liquid crystal display device. A PSA liquid crystal display device is produced by adding a small amount of a reactive monomer to a liquid crystal composition, introducing the liquid crystal composition to a liquid crystal cell, and polymerizing the reactive monomer in the liquid crystal composition by irradiation with an active energy ray. Accordingly, appropriate pretilt angles can be provided in the divided pixel, and, as a result, improved contrast brought about by improved transmittance and a high-speed responsiveness caused by a uniform pretilt angle can be achieved (for example, refer to PTL 1). However, in a PSA liquid crystal display device, a reactive monomer must be added to a liquid crystal composition and this has caused many problems in active-matrix liquid crystal display devices that require a high voltage holding capacity. There has been another problem of display failure such as ghosting.
A method with which the drawbacks of the PSA liquid crystal display devices can be overcome and a uniform pretilt angle is provided to liquid crystal molecules without contamination by foreign matter other than the liquid crystal materials in the liquid crystal composition has been developed. According to this method, a reactive monomer is mixed into an alignment film material and, after the liquid crystal composition is introduced into a liquid crystal cell, the reactive monomer in the alignment film is polymerized by irradiation with an active energy ray while applying a voltage between electrodes (for example, refer to PTL 2, 3, and 4).
As the size of the screen of liquid crystal display devices becomes larger, the method for producing liquid crystal display devices has undergone significant changes. That is, since the conventional vacuum injection method requires a long time to produce large-size panels, a one-drop-fill (ODF)-type production method has become the mainstream technology for producing large panels (for example, refer to PTL 5). Because this method involves a shorter injection time compared to the vacuum injection method, it has become the mainstream method for liquid crystal display device production. However, drop marks formed by dropping the liquid crystal composition remain in the liquid crystal display device while retaining their shapes even after fabrication of the liquid crystal display device. It should be noted that the drop marks are defined as a phenomenon that the trace left by dropping the liquid crystal composition appears as white marks in black display. The problem of drop marks occurs frequently in the aforementioned method of providing a pretilt angle to the liquid crystal molecules by adding a reactive monomer to the alignment film material because the reactive monomer, which acts as foreign matter during dropping of the liquid crystal composition onto the substrate, is present in the alignment film. In general, the occurrence of drop marks frequently depends on the choice of the liquid crystal material and the exact cause thereof is not clear.
A method for suppressing the occurrence of drop marks has been disclosed in which a polymerizable compound mixed into a liquid crystal composition is polymerized to form a polymer layer in the liquid crystal composition layer, thereby suppressing the occurrence of drop marks in relation to an alignment control film (for example, refer to PTL 6). However, this method has a drawback in that ghosting occurs due to the reactive monomer added to the liquid crystal composition and that the effect of suppressing drop marks is insufficient as with the PSA method and the like. Thus development of a liquid crystal display device with which ghosting and drop marks are less likely to occur while maintaining basic characteristics needed for liquid crystal display devices has been desired.